1. Field of the Invention
The present invention relates to thin film filters, and particularly to thin film filters having a negative temperature drift coefficient which can achieve better control of optical performance of an associated DWDM (dense wavelength division multiplexing) system when working within an operational temperature range.
2. Description of Related Art
In recent years, thin film filters have often been used in optical systems for signal processing or optical communications. The filters operate to select light of desired wavelengths, often within a narrow band. Thin film filters may be used in association with gradient refractive index (GRIN) lenses and optical fibers to form a dense wavelength division multiplexing (DWDM) device. Referring to FIG. 5, the operating principle of an eight-channel, filter type DWDM device is illustrated. Ideally, a light beam of a particular wavelength is considered one channel. In practice, one channel is defined by a very narrow range of wavelengths. The more channels a DWDM device has, the narrower the pass bandwidth of each channel.
To obtain narrower pass bandwidths, more layers of film are normally deposited on a glass substrate, creating a stack of films on the substrate. However, this procedure inevitably induces more internal stress in the film stack. The more tensile stress endured by a film stack, the looser the atomic structure of the films in the stack. Interfaces between film layers in the stack act as mirrors, which act to separate the wavelengths of a light beam. A looser atomic structure in a film stack lowers the reflectivity of these interfaces. Thus tensile stress in a film stack acts to broaden the pass bandwidth. Conversely, the more compressive stress endured by a film stack, the narrower the pass bandwidth of the filter is.
The coating process is designed to minimize pass bandwidth drift at room temperature (23° C.). The operational temperature range of a thin film filter is from −5° C. to 70° C. Within this temperature range, the stress in the filter varies substantially linearly with the temperature. FIG. 2 shows pass bandwidth of a conventional filter at room temperature. By way of example, Alcatel's 1915 LMI 10 mw WDM thin film filter has a positive temperature drift coefficient, 1 pm/° C. FIG. 3 shows how the pass bandwidth of Alcatel's 1915 LMI changes with a change in temperature. When the 1915 LMI's temperature increases from 23° C. to 70° C., a 47 pm pass bandwidth enlargement occurs, and when the temperature decreases from 23° C. to −5° C., a 28 pm pass bandwidth reduction occurs, as is illustrated in FIG. 3. Obviously, since temperature fluctuation and resulting pass bandwidth drift are inevitable, it is preferable if pass bandwidth is reduced more often than it is increased as the environmental temperature changes. Consequently, referring to FIG. 4, there is a demand for thin film filters having a negative temperature drift coefficient, in which pass bandwidth broadens when temperature decreases and narrows when temperature increases, as shown in FIG. 4. Note that in FIG. 4, the pass bandwidth increases less at the most extreme temperatures than for the Alcatel 1915 LMI case shown in FIG. 3.
Operational temperature fluctuation affects the stress present in a thin film filter, since film stacks and substrates of thin film filters are composed of different materials having different coefficients of thermal expansion (CTE). Thin film stacks are deposited on substrates under temperatures substantially higher than room temperature, and then are allowed to cool down to room temperature. If the CTE of a film stack is smaller than that of a substrate on which it is mounted, then the film stack will shrink less than the substrate does as they cool down. Therefore, a convex deformation occurs and a compressive stress is induced in the film stack (see FIG. 1b). This is the case of a stack-substrate combination having a negative temperature drift coefficient.
In nearly all of the prior art, DWDM thin film filters have positive temperature drift coefficients. This is the situation illustrated in FIG. 1a. Because the thin film stack is deposited under a temperature substantially higher than room temperature, when cooling down to room temperature, the film stack, which has a CTE greater than that of the substrate on which the film stack is mounted, shrinks more than the substrate does. Therefore, a concave deformation occurs. The film stack in this situation is under a tensile stress and pass bandwidth increases as temperature increases, which causes greater susceptibility to crosstalk as temperature increases. The tensile stress endured by the film stack is also a disadvantage during cutting operations, since it makes the affected film layers more brittle, increasing the probability of damage to the film stack during cutting. Furthermore, the adhesion between the film stack and the substrate may be overstressed, resulting in peeling of the film stack from the substrate.